Proc. 14th INA Conf., Reston VA, USA (Guest Ed. J. Self-Trail)
J. Nannoplankton Res. 35 (2), 2015, pp.111-128 © 2015 International Nannoplankton Association
ISSN 1210-8049 Printed by The Sheridan Press, USA

111

Silicoflagellates and ebridians from the Seto Inland Sea andKuroshio, including the description of Octactis pulchra var.
takahashii var. nov.
Kenta Abe*

Graduate School of Science & Engineering, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan; *s14e101d@st.yamagatau.ac.jp

Kevin McCartney

Department of Environmental Studies & Sustainability, University of Maine at Presque Isle, Presque Isle, ME 04769, USA

Yuko Fukunaga

Department of Earth & Environmental Sciences, Faculty of Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan

Hisashi Narita

School of Marine Science & Technology, Tokai University, Orido 3-20-1 Shimizu, Shizuoka 424-8610, Japan

Richard W. Jordan

Department of Earth & Environmental Sciences, Faculty of Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan
Manuscript received 27th March, 2015; revised manuscript accepted 24th June, 2015

Abstract Three silicoflagellate species, Octactis pulchra, Dictyocha stapedia and Stephanocha speculum, and theebridian Ebria tripartita, were observed in water samples from the Seto Inland Sea and the adjacent Kuroshio. Both
Octactis and Dictyocha exhibit two distinct morphologies, with skeletons made of thicker components predominatingin the Seto Inland Sea, and of thinner components in the Kuroshio. The skeletons of O. pulchra var. takahashii var.
nov. have a more fragile appearance, with thinner components, longer spines and a smaller apical ring, compared tobasal ring diameters, than the better known type variety O. pulchra var. pulchra. Both thick and thin forms of Octactis
and Dictyocha were often found as double skeletons, with skeletons of Octactis apparently attached at the abbasal
surfaces of the basal corners by organic matter, while those of Dictyocha were held in place by pikes. Only one specimen of S. speculum and a small number of ebridians were observed in the Seto Inland Sea, with both being absent in
the Kuroshio samples.
Keywords Japan, Kuroshio, Seto Inland Sea, ebridians, silicoflagellates, living assemblages, oceanography
1. Introduction
The Seto Inland Sea (also known as simply the InlandSea) is 450km long from east to west, and is relatively
shallow, with an average depth of 37.3m and a greatest
depth of 105m (Imai et al., 2006). It is almost entirely surrounded by Honshu, Shikoku and Kyushu Islands, but has
two open connections to the North Pacific via the Kii andBungo Channels, and a narrow connection to the Sea ofJapan via the Kanmon Strait (Figures 1a, b). In comparison to the Kuroshio, the waters of the Seto Inland Sea havea lower salinity and higher nutrient load associated with
dense human populations and fluvial input, notably fromthe Yodo, Takahashi, Kako and Ohta Rivers (Figure 1b).
The Seto Inland Sea is characterised by red tides (Okaichi,
2003), which have increased in frequency and scale sincethe 1960s, due to intense eutrophication along the coastline (Goto, 1975).
The Kuroshio is about 100km wide, 1km deep andsome 3000km in length (Barkley, 1970), and is charac-

terised by straight and meandering paths (see Figure 1a;
Kawabe, 1980). Fluctuations in these paths bring warmwater intrusions to Enshu Nada and Kumano Nada for different lengths of time, about 20 and 50 days, respectively(Kasai et al., 1993). Cold water rises to the surface between the Kuroshio meander and the Japanese coast, causing a temperature decrease of about 10˚C as the meander
develops (Barkley, 1970). The changing position of themeander results in fluctuating oceanographic conditions,which have a great effect on the distribution of the marine biota, notably the position of the fishing grounds. The
Kuroshio core-waters are generally warm (>23˚C), salty(>34.7‰) and low in nutrients (Lie et al., 1998; Du et al.,2013).
Only one significant report on silicoflagellates andebridians in the Seto Inland Sea exists in the literature.
Yanagisawa (1943) recorded Dictyocha stapedia Haeckel(as Dictyocha fibula var. stapedia (Haeckel) Lemmermann, and maybe some of the other subtaxa) in large num-

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bers in Osaka Bay, with Stephanocha speculum (Ehrenberg) McCartney & Jordan (asDistephanus speculum (Ehrenberg) Haeckel), Octactis pulchra Schiller (as Mesocena
polymorpha var. bioctonaria (Ehrenberg)
Lemmermann and Distephanus speculum
var. octonarius (Ehrenberg) Jørgensen), andthe ebridian Ebria tripartita (Schumann)
Lemmermann was also present. It should be
noted that the illegitimate name Distephanus
Stöhr was recently replaced by Stephanocha
McCartney & Jordan (Jordan & McCartney,2015).
During a search for silicoflagellate double skeletons (also called doublets) in a separate study (McCartney et al., 2014a), several illustrations of double skeletons of O.
pulchra were found in an unpublished thesison the Seto Inland Sea, written by Yuko Fukunaga. These samples, and others from theadjacent Kuroshio, have been re-examinedin the scanning electron microscope (SEM)
to determine the silicoflagellate and ebridian assemblage composition. Three silicoflagellate species were observed, including
double skeletons of Dictyocha and Octactis.
Examination of the Octactis double skeletons revealed new information on their finestructure and skeletal variability, and may
give clues to their evolution. In addition,
specimens of the rarely-studied ebridian E.
tripartita were observed and incorporated
into this study.

Figure 1: Seto Inland Sea and adjacent Kuroshio: (a) Map of sampling area. Isobaths
shown in metres. Approximate straight (black line) and meandering (white line) positions of the Kuroshio modified from Kawabe (1980). SIS - Seto Inland Sea, SoJ - Sea
of Japan, NP - North Pacific. (b) Map showing locations of rivers and oceanographic
areas mentioned in the text. KS - Kanmon Strait, BC - Bungo Channel, IN - Iyo Nada,
OR - Ohta River, HiN - Hiuchi Nada, TR - Takahashi River, HaN - Harima Nada, KR- Kako River, OB - Osaka Bay, YR - Yodo River, KC - Kii Channel, KN - Kumano
Nada, EN - Enshu Nada, SB - Suruga Bay. (c) Map showing location of Seto andBosei samples

2. Materials and methods
Surface-water samples were collected from,and south-east of, the Seto Inland Sea during the Bosei-maru Cruise (26th April to 2nd
May, 2008) of the Department of Ocean Science, Tokai University. The samples wereobtained using the shipboard sea-watersupply (for research use), with temperatureand salinity data recorded using a continuous salinity temperature recorder (Horiba
U-22XD water quality monitor). Two sets ofsamples were collected, Seto samples fromSilicoﬂagellates, ebridians, Seto Inland Sea, Kuroshio

the Seto Inland Sea and Bosei samples from the Kuroshio
and nearby Enshu Nada (Figure 1b). In total, 55 filter samples were examined for this study (Figure 1c): 28 samples
from the Seto Inland Sea, including three from southern
Shikoku (Seto-1 to Seto-6), four from the Bungo Channel (Seto-8 to Seto-16), four from Iyo Nada (Seto-18 to
Seto-27), four from Hiuchi Nada (Seto-29 to Seto-37), fivefrom Harima Nada (Seto-39 to Seto-51), four from Osaka
Bay (Seto-54 to Seto-63) and four from the Kii Channel(Seto-65 to Seto-75), as well as 27 samples in, or near to,
the Kuroshio, south-east of Shikoku Island and Kii Channel, including three from Kumano Nada (Bosei-31 to Bosei-33), three from Enshu Nada (Bosei-34+ to Bosei-36),
four from Suruga Bay (Bosei-1, Bosei-37 to Bosei-39) and
17 from the open ocean within the Kuroshio (Bosei-5 toBosei-26). A list of the sample data, water temperature,salinity, nutrients (silicate, nitrate, phosphate) and chlorophyll-a is provided in Table 1.
On board the ship, about 0.7-2.0 litres of surface seawater were filtered through a 47mm-diameter Millipore
HA-type nitrocellulose filter (0.45µm porosity). The filters
were dried at room temperature and stored in plastic Petri dishes. Later, in a shore-based laboratory at Yamagata
University, the filter samples were rinsed with distilledwater to remove salt crystals, and then re-dried and stored
again in plastic Petri dishes.
For the SEM study, samples were prepared by cutting
off a small piece of filter (3x3mm), which was mounted
on an aluminium SEM stub, then coated with Pt/Pd in an
Eiko IB-3 ion sputter-coater. The filter samples were thenobserved in a JEOL JSM-6510LV SEM. All silicoflagellates and ebridians on the entire filter piece were counted(Table 2), and many were photographed using the digital
camera built into the SEM (Plates 1-4). Most specimens
lay flat on their basal or apical rings, so that single skeletons were generally observed in apical or abapical views
and double skeletons in apical axis view. Some specimens
were oriented, or could be tilted, to provide lateral or
oblique views (see Figure 2 for the terminology used inthis paper).
Specimens were generally in excellent condition, although those of Seto-29 were abraded. Fragmented specimens were unusual, but those with more than half the basal
ring were included in the counts. The absolute abundances(number of skeletons/litre) of each taxon were calculatedusing the equation:

113

A = NS/V, where A = the absolute abundance, N = the number ofindividual skeletons counted on the stub, S = the scaling
factor and V = the volume of sea-water (in litres) filtered.
Double skeletons were counted as two individuals.
It should be noted that some specimens were photographed on either an extra stub of Seto-35 (Pl.3, fig.2), oran extra filter of Seto-59 (Pl.2, fig.4; Pl.4, fig.6), but thesewere not included in Table 2.
3. Results
A total of 256 specimens of Octactis were observed(Plates 1-3) and counted (Table 2), of which 229 (89%)
were O. pulchra var. pulchra and 70 (~27%) were double
skeletons, with all but five of the latter being from the Seto
Inland Sea. Of the Octactis specimens, 220 (86%) wereeight-sided, with nine seven-sided and 22 nine-sided examples (Table 2). There were also five doublet specimens,
composed of skeletons with a contrasting number of basal
and apical sides (herein labeled ‘mix’). Octactis can beseparated into two varieties, based on the thickness of thecomponents (elements or spines) that make up the skeletons.
A total of 57 specimens (representing 53 single andfour double skeletons) of Dictyocha stapedia were also
observed (Pl.4, figs 1-7) and counted (Table 2), of which
44 (77%) were found in the Seto Inland Sea. One specimen of Stephanocha speculum var. bispicatus (Bukry)
McCartney & Jordan and 15 specimens of E. tripartita
were found only in the Seto Inland Sea (Pl.4, fig.8 and figs
9-12, respectively). Observations on the doublets, as wellas on some interesting aberrant skeletons, constitute themain focus of this study. Some basic information on single
skeletons can be found in the Systematics section, below.
3.1 Observations on Octactis pulchra
double skeletons
In the study area, skeletons composed of both thick (Plates
1, 2) and thin (Plate 3) components of this species arepresent, but with different distributions. Skeletons madeof thicker components were mostly observed in the Seto
Inland Sea samples, while those with thinner components
occur more commonly in the Kuroshio samples. These
differences have prompted the proposal of a new variety,O. pulchra var. takahashii (see description in the System-

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Table 1: List of Seto and Bosei samples examined in this study and oceanographic data

atics section), for the skeletal morphology that has thinner components and often longer basal spines. Although
single skeletons were numerous, the samples also yieldeda large number of double skeletons, particularly of O. pulchra var. pulchra (Table 2; double skeletons:single skeletons = 65:159 in the Seto Inland Sea for var. pulchra, 3:20
in the Kuroshio for var. takahashii). In some cases, thecomponents of the daughter and mother skeletons are ofsimilar thickness (Pl.1, fig.3), whilst in others, the daughter skeleton has visibly thinner components (Pl.1, fig.5a;Pl.2, fig.2), particularly in the case of double-skeleton
specimens of O. pulchra var. pulchra. This difference inthickness is not restricted to the basal sides and spines, butalso to the elements that compose the apical ring (as seenin Pl.2, fig.1).
The double skeletons of Octactis are always in thecorner-to-corner configuration (see McCartney et al.,2015 for a discussion on configurations), but since theylack pikes, it is assumed that the abbasal surfaces of thetwo skeletons are held together by organic matter (cf. D.
stapedia). Most spines of the paired skeletons touch only
near the basal ring and separate distally, and thus are notgenerally in contact over their entire lengths, as is the casefor Stephanocha (McCartney et al., 2014a). There is also

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115

Table 2: Absolute abundances (in skeletons/l) and raw counts of the five silicoflagellate and ebridian taxa recorded in this study. Octactis and Dictyocha specimens further separated into single and double skeletons, and number of basal sides. In column of total skeletons (N), a double skeleton = 2
and a single skeleton = 1. V = volume of sea-water filtered116

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Figure 2: Terminology of silicoflagellate and ebridian skeletons, based on McCartney et al. (2014a) and Deflandre (1934), respectively. Octactis
pulchra var. pulchra in (a) apical and (b) abapical view. Dictyocha stapedia in (c) apical and (d) oblique abapical view. Ebria tripartita in (e) apicaland (f) oblique lateral view

Silicoﬂagellates, ebridians, Seto Inland Sea, Kuroshio

a gap between the basal rings along the length of the basalsides, with the gap being most pronounced opposite thestrut attachments (Pl.1, fig.4b; see also McCartney et al.,2014a). The cementing agent appears to be quite strong, asshown by one single skeleton in abapical view (Pl.2, figs
6a, b) that had a fragment of the basal corner of the otherskeleton still attached, after the rest had broken away. This
specimen was from sample Seto-29, in which there were
no intact double skeletons, presumably due to abrasionor partial dissolution, yet this specimen retained the connected fragment.
One interesting double skeleton (Pl.1, figs 5a-c) has amother skeleton in apical view nearest the observer and in
front of an incompletely formed daughter skeleton. The
mother skeleton has two apical ring elements, positioned
on opposite apical sides, which are incompletely formed,
with small gaps in the middle (see inset Pl.1, figs 5b, c).
The ends are closed, showing that these are not broken.
This provides further evidence that the apical ring elements form initially at the triple junctions and develop
towards the middle. This specimen also shows that themother skeleton may not be complete before the daughter forms. Two other specimens were observed with thisfeature on one apical side; in one case the gap is near the
triple junction, rather than in the middle.
3.2 Aberrants of Octactis pulchra
While truly distorted or unusual (see McCartney et al.,2014b) teratoid skeletons were rare, and are not presented
in this study, some minor aberrants are discussed here. A
few specimens possessed a slightly longer side with an additional, small spine, providing a very short, vestigial ninth
basal side that was not associated with a strut (Pl.2, fig.9).
Another specimen (Pl.2, fig.4) appears to have grown two
extra basal spines on the inner surface of the basal ring;
their bases seem to be continuous with the basal ring, andso do not appear to be remnants of a previously attachedskeleton of a doublet. Five doublets possessed skeletonswith different numbers of sides (‘mix’ in Table 2). Fourof these specimens (three in Seto-27 and one in Seto-51)
had eight- and nine-sided basal rings (e.g. Pl.2, figs 11a,b), while another specimen from Seto-51 had seven- andeight-sided basal rings, where one of the seven corners
was bypassed by a long side (Pl.2, fig.10). Two single
skeletons with bulbs at various locations on the basal ring(Pl.2, figs 4, 7), and a specimen that had a divided apical117

ring (Pl.2, fig.8), were also present in the Seto-51 sample.
Furthermore, single skeletons of the daughter lacking anapical ring were also observed and are the subject of aseparate study (McCartney et al., submitted). A similar
example has been illustrated already in a previous study(McCartney et al., 2014a, fig.6c).
3.3 Observations on Dictyocha stapedia
double skeletons
Specimens of D. stapedia were less abundant than Octactis
in most of the samples we observed. D. stapedia skeletons
made of relatively thinner (Pl.4, figs 1-3) and thicker (Pl.4,
figs 4-6) components were present in the study area, with
distributions similar to those of the O. pulchra thin andthick forms: i.e. the thin forms were mostly found in the
Kuroshio samples, while the thick forms were observed in
the Seto Inland Sea. However, apart from distribution anddegree of skeletal thickness, the morphology of the twoforms was identical. Of particular interest, though, are thedouble skeletons, which are always in the corner-to-cornerconfiguration and use pikes to hold the two skeletons together, in contrast to O. pulchra, which lacks pikes and ispresumably held together by organic material between thebasal corners (McCartney et al., 2014a). In some cases,
the daughter and mother skeletons are of similar thickness(Pl.4, fig.3), whilst in others, the thinner daughter skeleton
is easily visible (Pl.4, figs 6, 7). It is clear from Table 2 that
double skeletons are quite rare, compared to single skeletons (two versus 42 in the Seto Inland Sea, two versus 11
in the Kuroshio).
3.4 Absolute abundances ofsilicoflagellates and ebridians
Although silicoflagellate absolute abundances were quite
low, they were much higher in the Seto Inland Sea thanin the Kuroshio (up to 9.4x103 versus 0.5x103 skeletons/l,
respectively), particularly in the Osaka Bay area. With regard to the silicoflagellate and ebridian assemblage, Octactis was predominant and Dictyocha common in the Seto
Inland Sea (up to 7.8x103 versus up to 1.7x103 skeletons/l,
respectively), while both were uncommon in the Kuroshio(up to 0.4x103 versus up to 0.2x103 skeletons/l, respectively). On the other hand, ebridians and Stephanocha were
uncommon in the Seto Inland Sea, and not observed in the
Kuroshio samples.

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Plate 1
Octactis pulchra var. pulchra
single skeletons - 1, 2; double skeletons - 3-6(5b - magniﬁcation of upper right region of apical ring; 5c - magniﬁcation of lower left region of apical ring)abapical viewSeto-65

lateralview
Seto-49

apical viewSeto-27

apical view oblique viewSeto-43 40˚ tilt

apical viewSeto-27

apical viewSeto-25
1µm

10µm
1µm

Silicoﬂagellates, ebridians, Seto Inland Sea, Kuroshio

119

Plate 2Octactis pulchra var. pulchra
single skeletons - 3 (b - broken strut attachment), 4 (aberrant form with spines on bottom of basal ring, swollen basal sides), 5 (a- with broken struts, lacking apical ring; b - close-up of broken hollow struts), 6 (b - detail of corner of daughter(?) still attached), 7(aberrant form with basal ring expansion at corners of strut attachments), 9;
double skeletons - 1, 2, 8, 10 (near skeleton 7-sided, far skeleton 8-sided), 11

apical view
daughteron topSeto-51

apical view, daughter on Seto-43
top lacking
apicalring

Seto-27

apical view
daughterlacking
apicalring

2µm

abapical view apical viewSeto-59

Seto-292µm

abapical viewSeto-51

apical viewSeto-51

Seto-492µm

abapicalview
Seto-65

apical viewSeto-51

8- & 9-sided
pairunusualbridgeacross apical ringapicalview
0˚ tilt

abapicalview ofmotherextra
spine
not associatedwith strut
oblique view
45˚ tilt
Seto 27

10µm

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Plate 3Octactis pulchra var. takahashii var. nov.
single skeletons - 1, 3, 5, 6; double skeletons - 2, 4, 7 (type specimen)

apical view
Bosei-10

Seto-3510µm

abapical viewSeto-1

apical view
daughter lacking apicalringapical view
0˚ tilt
Seto-1

oblique view
40˚ tilt

apical view
Bosei-11
apicalview
Bosei-11
apical view
30˚ tilt

oblique view
40˚ tilt

Bosei-11

oblique view
0˚ tilt

Silicoﬂagellates, ebridians, Seto Inland Sea, Kuroshio

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Plate 4
Dictyocha stapedia, Stephanocha speculum, Ebria tripartita
single skeletons - 1, 2, 4, 5; double skeletons - 3, 6, 7 (thinner daughter skeleton in foreground)D. stapedia
apical view
Bosei-26

D. stapedia
obliqueabapical view
Bosei-26

D. stapedia
lateral view, 30˚ tilt
Bosei-31

5µm (1-3)

D. stapedia
obliqueabapicalview
Seto-49

D. stapedia
apical view

10µm (4-8)

D. stapedia
lateral view
30˚ tilt
Seto-59

Seto-49
D. stapedia
obliquelateralview
Seto-51

Seto-18
S. speculum apical viewE. tripartita
Seto-37

E. tripartita, Seto-27

E. tripartita, Seto-25

5µm (9-12)

E. tripartita, Seto-31

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4. Discussion
4.1 Double skeletons
Although many illustrations and photographs of O. pulchra exist in the literature, only a few of these show
double skeletons (e.g. Hovasse, 1932, pl.6, figs 15-17 asDistephanus octonarius (Ehrenberg) Haeckel; Ling &Takahashi, 1985, pl.1, fig.5 and pl.2, figs 1, 2, 4 as Distephanus pulchra (Schiller) Ling & Takahashi; Takahashi,
1991, pl.2, fig.7 as D. pulchra). McCartney et al. (2014a)
compared the double skeletons of O. pulchra and S. speculum (as D. speculum). Stephanocha, as shown in thespecimen illustrated in this study (Pl.4, fig.8), has the strut
attachment positions rotated sinistrally, when observed
in apical view, with the pikes being closer to the dextral
corner of each basal side. When observed in lateral view,
the basal side has a zig-zag design, because the elements
trend apically towards the strut attachment and abapicallytowards the pike attachment, which breaks the basal side
into three elements, two that connect to the basal corners
and a middle element between the strut and pike attachments (McCartney et al., 2014a).
O. pulchra does not have these features. The basalsides are composed of two elements, both connecting thebasal corners to the strut attachments, as there are no pikes.
Between the basal corners, the basal elements trend slightly in the apical direction, to produce a gap between thebasal sides of the skeleton pair when observed in obliqueor lateral view (Pl.1, fig.4b), but the basal ring is more flatand lacks the zig-zag design of Stephanocha. Octactis also
does not have the sinistral rotation of the apical structure
observed on most species of Dictyocha and all Stephanocha since the Late Miocene. There are other differences aswell. In general, the basal elements of O. pulchra var. pulchra are more massively built than in Stephanocha. The
basal spines also appear to narrow more quickly (compare
Pl.1, fig.2 with Pl.4, fig.8), and the spines of the paired
skeletons separate distally, where the abbasal surfaces ofStephanocha spines lie against each other. These distinctions are cited by McCartney et al. (2014a) as evidence
that Octactis and Stephanocha should be retained as separate genera.
4.2 Taxonomic considerations
Ling & Takahashi (1985) observed Octactis from several
locations, and concluded that O. pulchra should be included in Distephanus, since D. speculum subsp. tenuis

Bukry (now Stephanocha speculum var. tenuis (Bukry)
McCartney & Jordan) possessed a six-sided morphologythat has an apical ring made of thinner elements, similar
to those observed in O. pulchra. However, there are pronounced differences in the basal ring structures and strut
attachment locations of S. speculum and O. pulchra thatstrongly suggest that these are distinct genera.
In the present study, a few single skeletons and one
double skeleton of O. pulchra with an incomplete or broken apical ring were observed (Pl.2, figs 2, 3a, 5a), as wellas one specimen completely lacking struts (not shownhere). In other specimens, we have observed apical rings
with swollen parts near the middle of the apical side (seePl.2, fig.4), while another specimen forming a bridge
across the ring has a similar swelling in the middle of thebridge (see Pl.2, fig.8). This suggests to us that the apicalring elements form from both ends and subsequently meetin the middle. However, no double skeletons of Stephanocha with the daughter skeleton lacking an apical ring havebeen observed in this or our previous studies, and single
skeletons that have basal rings with incomplete struts arerare in both the modern and fossil records. This may suggest that Octactis and Stephanocha form apical structuresin a somewhat different manner.
In addition, an eight-sided skeleton of D. speculum var.
octonarius with an extra basal spine was illustrated by Gemeinhardt (1930, fig.59b), which appears similar to one
shown here as O. pulchra (Pl.2, fig.9).
4.3 Biogeography and ecology
In general, the Kuroshio was warmer, slightly moresaline and with fewer nutrients than the Seto Inland Sea(Table 1, Figure 3), with a particularly significant drop in
salinity in Osaka Bay due to inflowing rivers. These rivers were probably responsible for transporting nutrients to
Osaka Bay, where eutrophication is known to be a problem (Hoshika et al., 1998). The nutrient data in Table 1 andFigure 3 show higher silicate concentrations in the Seto Inland Sea than in the Kuroshio (up to 15 versus 1-3µmol/l,
respectively), particularly in Osaka Bay, whereas those ofnitrate and phosphate remained low throughout the cruise(except at Bosei-35, where silicate reached 15µmol/l andnitrate reached 10µmol/l). The absolute abundance data
for O. pulchra var. pulchra and D. stapedia show higher values in and around Osaka Bay, where there is alsoa peak in chlorophyll-a (Figure 3), seemingly caused by

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123

Figure 3: Temperature, salinity, nutrient concentration, chlorophyll-a and absolute abundances of the five taxa recorded in this study (in skeletons/l),
with stations arranged in chronological order. Physico-chemical data downloadable from http://kutty.og.u-tokai.ac.jp/~kunio/2008kaijitsu/2008og_
kaijitsu3.htm

dinoflagellates and the centric diatom Skeletonema (Fukunaga, unpubl. obs., 2009), with the latter species often
responsible for red tides in the bay (Oshima et al., 2009).
Relatively higher silicoflagellate abundances were also recorded by Yanagisawa (1943) from Osaka Bay, but skeletons of Octactis pulchra were probably misidentified as
Mesocena and Distephanus in his counts, making directcomparisons difficult.
The ecological conditions that might be responsible
for the O. pulchra skeletal variations that have relatively
thick or thin components is not easy to identify, but peaks

in temperature tend to equate with higher numbers of O.
pulchra var. takahashii and the thin form of D. stapedia,
suggesting a change in oceanographic conditions (increased temperature combined with low silicate values)
is associated with the morphological change. In addition,
many of the aberrants were found in Samples Seto-51
and Seto-59 in Osaka Bay, which is characterised by low
salinities caused by river inflow, but also high silicates,
high diatom and dinoflagellate abundances, as well as thehighest silicoflagellate abundances. Interestingly, Tsujita(1949) reported a red tide of Dictyocha fibula (presumed

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Abe, McCartney, Fukunaga, Narita, Jordan

herein to be D. stapedia) in Omura Bay in Kyushu during
August, caused by lowered salinities due to the mixing ofriver- and sea-waters. In the red tide, some of the skeletonswere aberrants, echoing our observations. Clearly, further
studies of this type need to be carried out in order to draw
some conclusions on whether skeletal plasticity is related
to environmental stress.
5. Systematics
Class DICTYOCHOPHYCEAE Silva, 1980
Order DICTYOCHALES Haeckel, 1894
Family DICTYOCHACEAE Lemmermann, 1901a
Octactis pulchra Schiller, 1925 var. pulchra
Pl.1, figs 1-6; Pl.2, figs 1-11
Description: Skeletons robust, with circular (Pl.1, figs
4-6), or slightly elongate (Pl.1, figs. 1, 3) basal rings of 7-9
basal sides and with length:width ratios of up to 1.15:1.
One basal spine at junction of two basal sides. Usually,
one pair of opposing basal spines longer than the others(e.g. Pl.1, fig.1). Pikes absent. Generally, strut attachments
at midpoint of each side, but not rotated sinistrally in apical view, as with modern Dictyocha and Stephanocha(McCartney et al., 2014a). Struts unusual, as thickness
diminishes markedly from strut attachment towards apicalring, with apical ring elements being thinner (Pl.1, fig.2).
Sometimes, apical ring elements offset (Pl.1, fig.3), orwith expanded region or knob (Pl.2, fig.8) near midpointof element. These features are not apical spines, but apicalspines may occur in Southern Ocean (Hallegraeff, pers.
comm., 2013) and Mediterranean Sea (Malinverno, pers.
comm., 2013). Dimensions: Major axis outside diameters of basal ring ~35µm; major axis spines ~10µm long,~2.5µm thick at basal ring; minor axis spines ~80% ofmajor axis thickness and length. Strut ~5µm long, ~2µm
thick near basal side, about a quarter that thickness nearapical ring. Apical ring elements up to 1µm thick. Occurrence: Abundant to uncommon in the Seto Inland Sea(Seto-4, 12, 18, 25, 27, 29, 31, 37, 41, 43, 49, 51, 54, 58,
63, 65, 71), although occasionally absent. Largely absent
in the Kuroshio, sometimes uncommon (Bosei-32, 34,
35, 36). Previous records: The type variety, var. pulchra,has been generally associated with coastal enclosed areas,such as the Gulf of California (Murray & Schrader, 1983;
Martínez-López et al., 2012), Seto Inland Sea (Yanagisawa, 1943) and the Adriatic Sea (Rigual-Hernández etal., 2010), and has been observed off the coast of Panama(Takahashi, 1991). However, this variety has also beenrecorded in sediment traps in open-ocean equatorial locations in the Pacific and Atlantic Oceans (Takahashi, 1991),
as well as in the subarctic Pacific (Onodera & Takahashi,2007) and in an Atlantic Ocean warm-core ring (Takahashi& Blackwelder, 1992). In addition, it has been found in
Holocene sediments in the Mediterranean (Dumitricǎ,1973), off California (Barron & Bukry, 2007a), in the Gulf
of California (Barron et al., 2004, 2005; Barron & Bukry,
2007b), and in deep ocean sediments of the equatorial Pacific Ocean by Bukry & Foster (1973), Bukry (1976, 1978,
1980, 1982a, 1983) and McCartney et al. (1995).
Octactis pulchra var. takahashii McCartney, Abe &Jordan var. nov.
Pl.3, figs 1-7. Derivation of name: After Professor Kozo
Takahashi (Hokusei Gakuen University, Japan), micropalaeontologist, in recognition of his earlier observations on
this taxon. Diagnosis: Skeletons, fragile-looking, with circular to slightly elongate basal ring, composed of 7-9 basalsides of varying length. Strut attachments near middle ofeach side. Basal spines long, narrow and curved in various directions. Basal spine length may be equal to basal
ring diameter. Apical ring often with different symmetry
to basal ring; apical ring may be same size as basal ring,or appreciably smaller. Apical ring elements appreciably
thinner than basal elements. Differentiation: Comparedto var. pulchra, the skeleton of the new variety is smaller (outside basal ring diameters of 21.0-25.5µm versus
28.3-33.3µm in var. pulchra in this study), more fragilelooking (i.e. with relatively thinner basal spines and sides)
and with much longer basal spines (lengths of 8.3-18.9µm
versus 6.7-12.8µm in var. pulchra in this study). In somespecimens, the basal spines are sometimes strongly curved(Pl.3, figs 4a, b), while in others they are straight (Pl.3, figs
2, 3). Dimensions: Type specimen with basal ring majoraxis outside diameter of 24.3µm, minor-axis diameter of
21.5µm, length:width ratio of 1.13. Longest basal spine= 14µm. Strut length ~4µm. Holotype: Pl.3, figs 7a-c
(double skeleton in apical view). Paratype: Pl.3, fig.6(single skeleton in apical view). Type locality: Bosei-11(32.498˚N, 138.191˚E), in the Kuroshio, south of Japan.
Repository: Since it is difficult to preserve stubs and filters, the holotype is an illustration, in accordance with
Article 40.5 of the International Code of Nomenclature

Silicoﬂagellates, ebridians, Seto Inland Sea, Kuroshio(McNeill et al., 2012). A digital image file will be deposited in the National Science Museum, Tokyo. Occurrence:
Uncommon in the Kuroshio (Bosei-9, 10, 11, 17, 20, 31,32), but sometimes absent. Largely absent in the Seto Inland Sea, although sometimes rare (Seto-1, 8). Previous
records: Takahashi (1991) illustrated five specimens of O.
pulchra (as Distephanus pulchra (Schiller) Ling & Takahashi), one of which was a double skeleton (his pl.2, fig.7),
while another had 10 basal sides, with apical sides bearing small spines (his pl.2, fig.10). In addition, a specimen
from the Indian Ocean was illustrated by McCartney etal. (2014a, fig.6B). A further two specimens from a Gulf
Stream warm-core ring, observed by Takahashi & Blackwelder (1992) using the light microscope, may also belong
to this new taxon, although the basal spines cannot be seenclearly enough to be certain.
Dictyocha stapedia Haeckel, 1887
Pl.4, figs 1-7
Description: The bridge is relatively long and canted sinistrally. Looking down the length of the bridge in apicalview, the sinistral strut is longer and attaches further fromthe major axis basal corner than the dextral strut. There isa pronounced apex spine, and there may be small apicalspines on the flanks of the struts, which may occur inconsistently and vary in location on all struts. Prominent pikes
occur on all basal sides. Four double skeletons, including a mother-daughter doublet (Pl.4, fig.7), were counted,
whilst another double skeleton, also as a mother-daughter
doublet (Pl.4, fig.6), was found on a stub made from another piece of filter, but not counted. The double skeleton
featured in Pl.4, fig.3 is almost identical to that illustrated
by Haeckel (1887, pl.101, fig.12). Differentiation: Dictyocha mandrai Ling possesses accessory spines on theoutside of the middle part of the basal ring, while D. messanensis apparently lacks pikes (“Stützstacheln bisher
nicht beobachtet” in Lemmermann, 1901b, p.261), thus
distinguishing them from both the thick and thin forms of
D. stapedia. Haeckel (1887, p.1561) also mentions thatD. stapedia is “similar to the hats of Dictyocha fibula andDictyocha messanensis, but distinguished by four small
centripetal teeth, which start from the inside of the basal
ring at the side of the four ascending beams”, implyingthat D. messanensis lacks pikes. Occurrence: Common
in the Seto Inland Sea (Seto-1, 4, 16, 31, 43, 49, 51, 54,
58, 63, 65), although occasionally absent. Uncommon in125

the Kuroshio (Bosei-10, 11, 16, 17, 20, 24, 26, 31, 32, 33,35), but sometimes absent. Previous records: This species is often recorded in deep-sea drilling cores, but rarely in water samples, despite being described from water
samples from the Atlantic, Pacific and Indian Oceans by
Haeckel (1887). The specimens observed in this study are
consistent with those identified as D. stapedia by Loeblich
et al. (1968), while those by Poelchau (1976), Murray &Schrader (1983), Onodera & Takahashi (2007) and McCartney et al. (2014a) identified as D. messanensis, andmany others identified as D. fibula in the biological literature, are actually D. stapedia.
Stephanocha speculum var. bispicata (Bukry, 1982b)
McCartney & Jordan in Jordan & McCartney, 2015
Pl.4, fig.8
Only one specimen was observed in this study. Occurrence: Rare in the Seto Inland Sea (Seto-18). Previous
records: Although described from Upper Miocene to Upper Pliocene sediments of the eastern equatorial Pacific(Bukry, 1982b), it has been recorded in Holocene sediments from the North Pacific (Poelchau, 1976) and as extant in Southern Ocean surface waters (Malinverno, 2010).
This taxon is therefore in need of further investigation.
Class THECOFILOSA Cavalier-Smith in CavalierSmith & Chao, 2003
Order EBRIIDA Deﬂandre, 1936
Family EBRIACEAE Lemmermann, 1901a
Ebria tripartita (Schumann, 1867) Lemmermann, 1899
Pl.4, figs 9-12
Description: Internal siliceous skeleton triradiate, with Yshaped triode, composed of three thick elements (a rhabde
and two actines). Each actine bifurcates into slightly thinner elements, the proclade and opisthoclade, which diverge
in opposite directions. The two proclades bifurcate again
into synclades, which connect to an apical ring. The openings between the actines, proclades, synclades and apicalring are called superior windows. The two opisthoclades of
the actines bifurcate, with one branch joining the base of
the rhabde and the other forming an abapical ring. Anotheropisthoclade arises from the base of the rhabde and joins
the apical ring. The openings between the rhabde, opisthoclades and abapical ring are called inferior windows.
Short spines often adorn some of the thinner elements. Allelements are solid. Dimensions: According to Hargraves

126

Abe, McCartney, Fukunaga, Narita, Jordan(2002), skeleton is 24-34µm in length, 22-31µm in width,with a length:width ratio of 1.1. Specimens in the present
study are 28.2-32.0µm in length, 23.0-24.5µm in width.
Occurrence: Only low abundances (<400 skeletons/l)
were recorded in the present study. Uncommon in the Seto
Inland Sea (Seto-25, 27, 29, 31, 37, 41, 58), or absent. Absent in the Kuroshio. Previous records: Absolute abundance rarely more than 104 cells/l (Hargraves, 2002), buthigher abundances have been recorded during the summer
in Long Island Sound (4.9x104 cells/l: Conover, 1956) and
off Rhode Island (Ernissee & McCartney, 1993). Usually,E. tripartita is found in cold to warm, temperate coastal
localities (e.g. Vancouver, Canada: Hoppenrath & Leander, 2006; coastal Brazil: Gomes et al., 2007). However,Vørs (1992) found this taxon in the Gulf of Finland, while
Onodera & Takahashi (2007) reported this taxon as rare in
their sediment-trap (50˚N) study in the subarctic Pacific.
6. Conclusions
The specimens of O. pulchra provided an opportunity to
study its fine structure and morphological variation, particularly for double skeletons, but also for aberrants. The
species is unusual, compared to modern Dictyocha andStephanocha, in that the basal rings are made of sides
that are of more linear shape and lack pikes. More fragile-looking specimens of O. pulchra with longer spines
were found in the Bungo Channel entrance and outside
the Seto Inland Sea. These specimens are described as
O. pulchra var. takahashii var. nov. Thinner skeletons of
D. stapedia were also observed in the Kuroshio, but theirmorphology was identical to that of the Seto Inland Sea
specimens, so a new variety was not warranted. In thisstudy, double skeletons of Octactis were quite numerous,
while some D. stapedia doublets were also present. Further observations on mother-daughter skeletal differencesin doublets of both genera corroborated previous findings,
and provided further examples of what is still essentiallya poorly-documented phenomenon. Many of the aberrantsof Octactis documented herein by SEM images were only
known from drawings in the old literature. Comparison of
the absolute abundances and distributions of the thick andthin types of O. pulchra and D. stapedia, as well as thoseof the aberrants, suggests that changes in the skeletal morphology may be related to increased temperature (thintype skeletons) and decreased salinity, high silicate concentrations and increased productivity (aberrants). These

environmental studies warrant further investigation, bothin the Seto Inland Sea and in other areas (e.g. the Southern
Ocean), where changes in skeletal thickness and aberrants
are known to occur.
Acknowledgments

We thank former Yamagata University students Susumu Konno,
Yumi Arai and Shinichi Saito for collecting and processing thewater samples used in this study. Hideto Tsutsui provided assistance in various ways during the writing of this paper. Photographs of Octactis specimens from the Southern Ocean andMediterranean Sea were provided by Gustaaf Hallegraeff andElisa Malinverno, respectively, for comparison with specimens
observed in this study. This research was done during a Trustees
Professorship granted to KM by the University of Maine system. Finally, we would like to thank Elisa Malinverno and HelgeThomsen for their useful and constructive reviews of the manuscript, as well as suggestions made by Jean Self-Trail and Jamie
Shamrock.

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